Method for oxidizing the surface of recuperator tubes



United States Patent ABSTRACT OF THE DISCLOSURE A method of treating the surface of recuperator tubes with a hot oxidizing gas where the tubes have been built into the flue channel of a recuperator, the hot oxidizing gas being a fiue gas from a furnace operating with an excess of air to form a uniform protective oxide layer on the recuperator tubes.

This invention relates to a method for treating the surfaces of recuperator tubes.

It has been found that in operation the behavior and the resistance of recuperator tubes to temperatures do not depend entirely on the precise maintenance of their walls at a temperature on which their composition is based,

but rather that the condition of the surface of the tubes has decisive significance, particularly as far as the life of the tubes is concerned.

It has been a well-known practice to provide metal surfaces with an oxide layer by exposing them at high temperatures to an oxidizing atmosphere. It has also been found that particularly in recuperators which become excessively hot, the surface treatment of the tubes, that is, the formation of a uniform, firmly adhering and dense layer of oxide, has great significance, in view of the different conditions and properties of the material from which the tubes are made.

My invention comprises a method for treating the surface of recuperator tubes by which a defined condition is produced on the surface of the tube material as deliverecla condition which remains unaffected by different and varied operating conditions. For this purpose, the invention provides that before the outer or exterior Walls of the tubes are reacted upon by the flue gases, the tubes on the exterior surfaces of their Walls.

It is preferable that while the hot oxidizing stream of gas flows around and in contact with the outer Walls of the tubes in the flue-gas channel, an oxidizing stream of gas is allowed to flow through the interior of the tubes at the same time. To be sure, during operation, contrary to the conditions prevailing on the exterior surfaces of the tubes, there is an oxidizing atmosphere sufiicient to oxidize the tubes prevailing in the interior of the tubes, but in general the optimum temperature and time conditions for the formation of .a close, uniform oxide layer are not present in the interior of the tubes. In the method of the invention, it is preferable to combine the formation of the protective oxide layer on the interior and on the exterior together into one procedure or operational step by so synchronizing or attuning the wall temperature and the reaction time of the oxidizing atmosphere with the material composition of the tube that favorable conditions will be present for the formation of both layers, that is, the inner layer and the outer layer. The method of the invention has the advantage of not having to subject the inner and outer surfaces of the tubes to any special 3,377,213 Patented Apr. 9, 1968 prior to their installation in the fiuegas channel; instead, the available operational means, namely, the flue-gas channel, the air, and the heat producing means, are used to undertake the surface treatment at the place of installation.

The temperature, volume, and thus the velocity of flow of the gas stream flowing through the interior of the tubes may also be variable. This allows one to vary the wall temperature in such a manner as to obtain an oxide layer on the inner or the outer side of the tubes of the desired consistency.

In a recuperator which is connected to the outlet side of a pit furnace or soaking pit, when the tubes have been pretreated in accordance with the present invention, the outer Wall is acted upon by the hot, oxidizing stream of gas in the flue-gas channel after the furnace hasdried out, and by synchronizing or attuning to one another the throughput of the hot oxidizing stream of gas With the throughput of the air through the recuperator tubes, it is possible to obtain the desired temperature of the atmospheres flowing around and through the tubes, and thereby the desired wall temperature.

Two examples of steel from hich the recuperator tubes are made are as follows:

surface treatment Example 1 Percent Example 2 0.15 to 0.25 0.5 to 1.3

24.0 to 27.0 Ni (optional) 3.5 to 5.5

N (optional) 0.10 to 0.20

The protective layer of oxide formed on the tubes made from the foregoing steels is chromium oxide. My method produces such layer on the exterior and/or interior surfaces of the tubes Whether the surfaces thereof are clean, bright and shiny, or Whether they are covered with scale.

Some examples of the hot oxidizing gases which I use to produce the oxide layer on the external and internal surfaces are as follows:

Flue gas from coke oven gas or long-distance gas burned with a excess of air:

Percent N 71.4 H O 18.5 3.7

Flue gas from open hearth gas burned with a excess of air:

Percent CO 19.8

lue gas from fuel oil burned with a excess of air:

Percent The foregoing examples of the hot oxidizing gas represent minimum amounts of excess air used in preparation of same, so that the amounts of excess air employed can be increased from those identified to obtain satisfactory oxide layers.

In formation of the oxide layer on the exterior surfaces of the tubes, the temperature of the oxidizing gas flowing around and in contact with the exterior surfaces of the tubes is 900 C. to 1200 C., at a rate of 0.5 to 12 'm./sec. Production of the oxide layer in the interior surfaces of the tubes results from flow of air to be used in combustion of a fuel or of an oxidizing gas through the tubes at a temperature of up to substantially about 700 C. to 1000". C. and at a rate of 0.10 to 8 rn./sec.

In one embodiment of my method, formation of the oxide layer on the interior surface of the tubes results from passing the air to be used in combustion of the fuel through the tubes while subjecting the exterior surfaces of the tubes to the hot oxidizing gas. In this practice, the temperature of the air flowing through the tubes is up to about 700 C. and its velocity is 0.10 to 8 m./ sec.

During formation of the oxide layers, the temperature of the walls of the tubes reaches about 800 C. to 1000 C.

While I have described preferred embodiments of my invention, it may be otherwise embodied within the scope of the appended claims.

The time during which the treatment of the tubes has to be carried out ranges from one to six hours.

The higher the temperature and the rate of the gas flow around the tubes, the lower may be the temperature of the air or the oxidizing gas passing through the interior of the tubes and, in the limit case, it may be identical with the temperature of the ambient atmosphere.

I claim:

1. In a method of operating a recuperator in connection with a furnace from which flue gases are discharged into said recuperat-or at a temperature of at least about 1000 C., the improvement comprising operating said furnace with at least 25% excess air in said flue gases for a short period of time to form a protective layer of oxide on the surfaces of said recuperator elements prior to normal operation of said furnace.

2. In a method for oxidizing surfaces of recuperator elements with'hot oxidizing gases at a temperature of at least about 1000 C., the improvement comprising contacting the surfaces of said recuperator elements with hot flue gases from a furnace having at least 25% excess air for a period sufilcient to oxidize said surfaces prior to the normal operation of said recupe-rator.

3. The improvement of claim 1 wherein said recuperator includes a plurality of tubes, including directing a flow of air through said tubes.

4. The method of claim 3 characterized by said air directed through said tubes having a temperature in at least a portion of said tubes of substantially about 700 C. to 1000 C., said hot oxidizing gas heating the walls of said tubes to a temperature of substantially about 800 C. to 1000" C.

5. The method of claim 4 characterized by said hot oxidizing gas flowing around and in contact with the exterior surfaces of said tubes at a rate of substantially about 0.5 to 12 -m./sec., and by said air flowing through said tubes at a rate of substantially about 0.10 to 8 m./sec.

6. The method of claim 1 characterized by treating said surfaces of said elements after a furnace to which said recuperator is connected has been dried out, and by directing said hot oxidizing stream through a flue-gas channel of said recuperator.

References Cited UNITED STATES PATENTS 2,834,581 5/1958 Schefels et a1 165134 2,865,750 12/1958 Trevoy 1486.35 2,937,855 5/1960 Hazen l134 3,125,471 3/1964 Conner 148-635 OTHER REFERENCES Trinks: Industrial Furnaces, vol. I, 3rd ed., Wiley 8: Sons, New York (1934), p. 360.

ALFRED L. LEAVITT, Primary Examiner.

J. H. NEWSOME, Assistant Examiner. 

